The present invention relates generally to telecommunication transmission facilities and, more particularly, to a maintenance shelf system having a digital network interface and a common control unit with a protection switching mechanism. Many telecommunication transmission facilities include a central office which may transmit data, or "payload," signals over transmission lines to customer equipment on the customer's premises. Typically, digital payload signals are sent over the transmission lines to a digital network interface. These may also be referred to as a Network Interface Unit ("NIU").
The NIU is the demarcation between the telephone operating company's side of the telephone line and a customer's side of the telephone line. Electrically, the NIU is generally transparent to payload signals, but can be used for special maintenance functions, such as loop back. A channel bank receives signals from the NIU and converts the payload signals to analog signals. The channel bank transmits an analog signal for each channel differentially on two wire conductors, known as a Tip-Ring pair.
The Bell telephone system in the United States, for example, has widely utilized digital time-domain multiplexing pulse code modulation systems. This is known as a T-1 transmission system. Each T-1 transmission system carries 24 8-KB/second voice or data channels on two pairs of exchange grade cables. One pair of cables is provided for each direction of transmission. T-1 transmission systems are used in multiples 37 N", providing 37 N"-times-24 channels on 37 N"-times-two cable pairs. The cables exist in sections, called "spans," between and beyond a series of regenerative repeaters.
Payload signals are received by the telephone company and are transmitted, via the first spans of transmission lines, to alternating series of regenerative repeaters and further spans. Regenerative repeaters are spaced approximately every 6,000 feet, and the "spans" are the lengths of the cables between the repeaters. Each repeater receives data from a preceding repeater (or in the case of the first repeater, the central office) via a span on either side of the repeater. Each repeater generally recognizes pulses on the spans, however degraded, and regenerates clean pulses on the next spans.
The repeaters and cable spans continue until the lines extend to the NIU. From the NIU, customer connections continue into the customer premises.
A spare set of transmission cables, or a "spare line," is desirable, to provide for events of natural disaster, accident, maintenance, and upgrade. A spare transmission line is often provided to permit switching the spare transmission line into service in substitution for any one of the 37 N" multiple service lines which might become disabled. This is known as 1:N protection switching.
Since each T-1 span requires a network interface unit and multiple spans are typically routed together, multiple network interfaces are placed in the same physical location. The mounting that provides access to the network interface is called a maintenance shelf. Maintenance shelves are also known to provide a common control unit that serves as a remote control for the switching of a spare line in a 1:N protection system. Remote control of the span switching process reduces the need for technician visits to maintenance shelves located miles from telephone company central offices.
It is known to provide such a protective switching mechanism in which the mechanism monitors the inoperative line for a "restore service" signal and returns the service and spare lines to the original condition in response to such a signal. Unfortunately, while advantageous, such switching mechanisms do not as completely eliminate technician visits to maintenance shelves as would be desired. U.S. Pat. No. 5,224,149 is incorporated by reference, for additional illustration of the state of the art.